The present invention is directed in general toward local area networks and, more particularly, toward an improved method for transmitting message signals in a local area network.
Continued progress in integrated semiconductor circuitry has led to widespread usage of autonomous computing devices such as personal computers, printing devices, storage devices, automated manufacturing devices, etc. With this increase in usage of personal computing devices comes the need to provide intercommunication among these devices. Hence, computing networks have been developed to allow one user of the network to exchange communications and/or data with other users of the network. These networks require communication protocols to allow exchange of the aforesaid information and/or data in the most expedient and efficient manner.
Prior art protocols for exchanging data among computing devices of a local area network fall generally into one of two categories, to wit, token passing or contention.
Generally, token passing protocols employ a token message signal which is passed from station to station within the network, each station being permitted to transmit message signals only after receiving the token message signal. Thereafter, each station transmits the token signal such that another station is permitted to transmit its message signals. These systems, however, incur unacceptable delays during periods where few stations have message signals to transmit and, therefore, those few stations having message signals to transmit must wait until the token signal is passed among the plurality of stations not having messages to transmit before they may gain access to the transmission medium.
Contention local area networks are those in which each station may gain access to the transmission medium to thereby transmit its message signal at any time the subject station determines the transmission medium is idle. When more than one station attempts to gain access to the transmission medium simultaneously, i.e., a collision occurs, each station will generally discontinue its transmission for a randomly selected time interval and, thereafter, re-attempt to gain access to the medium. These systems, however, incur unacceptable delays at times when many stations have message signals to transmit and, therefore, many collisions occur.
An improved contention system, the preemptive BID Communication System, disclosed in U.S. Pat. No. 4,517,670, provides a contention protocol wherein only those stations involved in a collision, i.e., preempted stations, are allowed to transmit following the collision. In accordance with the preemptive BID protocol, a communication period is provided after a collision. During the communication period preempted stations follow a method which allows them to transmit message signals in accordance with their physical location along the network transmission medium. While the preemptive BID protocol works well in networks where the time to transmit a message signal is on the order of, or greater than, the communication period (typically chosen to equal the two-way propagation delay of the network transmission medium), the system introduces considerable inefficiences and waste in bandwidth in high speed networks where the time to transmit a message signal is less than the communication period.
Therefore, it is desirable to provide a protocol for transmitting message signals in a local area network which protocol works as efficiently during times when few stations have messages to transmit as it does during times when many stations have messages to transmit. Further, it is desirable to provide such a system which is efficiently used in high speed applications.